Our overall objective is to determine the relationship between the structure and function of the snake venom neurotoxins. The dominant group of these neurotoxins produce a non-depolarizing curare-like block of neuromuscular transmission in a post-synaptic mode. The toxin inhibits acetylcholine receptor. It is an antagonist. Now that we have established the three-dimensional structure of one toxin, erabutoxin b, and determined the probable reactive site, our proposed work will include the following: 1) Refinement of erabutoxin b structure to the limit of data less than 1.8 angstrom units using either new heavy atom derivations and ultimately the Diamond refinement procedure or the phase refinement method of Sayre; 2) Determination of the structure of erabutoxins a and c, laticotoxin, Laticauda semifasciata II, and of at least one neurotoxin of the 72 residue series; 3) Determination of the structure of the acetylcholine receptor - toxin complex. BIBLIOGRAPHIC REFERENCE: Preston, H. S., Kay, J., Sato, A., Low, B. W., and Tamiya, N., Crystalline Erabutoxin c, Toxicon, 13, 273, 1975. Low, B. W. (1975) Structure Studies of a Sea Snake Neurotoxin "Erabutoxin b", Symposium Lecture, Jubilee Meeting Japanese Biochemical Society, J. Biochem. (Tokyo) in press.